Pennsylvanians worried about the effects of natural gas drilling may have another reason for concern. Homes closest to fracking zones have been exposed to rising levels of cancer-causing radon ever since the state’s drilling boom began in 2004, a new public health study found. The analysis is part of a growing body of research that explores how widespread shale gas production affects the air, water and human health -- and what, if anything, should be done to limit the impacts.

Radon is a naturally occurring radioactive gas found in rocks and soil. While the gas seeps through cracks in home foundations and local water wells on its own, the disruptive process of fracking frees up even more radon and other potentially harmful materials and brings them to the surface. After s...